Megga test motor driven of softstarter

ALWAYS disconnect the controller's LOAD side wiring prior to verifying the motor's windings insulation with a high-voltage tester (megger).

dmargineau said:

ALWAYS disconnect the controller's LOAD side wiring prior to verifying the motor's windings insulation with a high-voltage tester (megger).

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Thanks for replay
Can I use low voltage on meggar as 220Volt to meggar 460V motor without disconnect?

strujic said:

Thanks for replay
Can I use low voltage on meggar as 220Volt to meggar 460V motor without disconnect?

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Are you saying test the insulation at 220v?

What would be the point?

I suggest that at 460v, you should set the range to 1000v after disconnecting the soft starter load cabling.

markie said:

Are you saying test the insulation at 220v?

What would be the point?

I suggest that at 460v, you should set the range to 1000v after disconnecting the soft starter load cabling.

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For quick test any motor from 50HP to 200HP with isolation readings above 5 Mohm will be satisfactory. I have to quick inspect 20 motors and do not have time to disconnect all of them.
I know for accurate reading we set tester to 500V and take a proper readings. With my expirience if motor is bad you will see with any voltage on meggatester.

strujic said:

For quick test any motor from 50HP to 200HP with isolation readings above 5 Mohm will be satisfactory. I have to quick inspect 20 motors and do not have time to disconnect all of them.

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In my experience the motors MUST be disconnected.

If you do a test with everything connected and you get a bad result, how do you know that it is the motor and not the cable.

I remember an experienced electrician testing a motor which he didn't disconnect, but did isolate the motor with the local isolation switch. He got a bad test result so changed the motor. New motor, same problem. Turned out the cable was bad.

I always, always, always disconnect a motor before testing.

Mark

I you have soft starters and you don't disconnect them from the load and use a megger you are going to blow them up even at 220 or 500 volts I promise you.

Take the time and do it correctly.

The Plc Kid said:

I you have soft starters and you don't disconnect them from the load and use a megger you are going to blow them up even at 220 or 500 volts I promise you.

Take the time and do it correctly.

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thanks this is all I wont

MarkNightingale said:

In my experience the motors MUST be disconnected.

If you do a test with everything connected and you get a bad result, how do you know that it is the motor and not the cable.

I remember an experienced electrician testing a motor which he didn't disconnect, but did isolate the motor with the local isolation switch. He got a bad test result so changed the motor. New motor, same problem. Turned out the cable was bad.

I always, always, always disconnect a motor before testing.

Mark

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He is not electrician maybe cooky

I always meggar motor from start panel and get readings of motor and cable. If readings not good open motor terminal box disconnect cable and meggar motor windings separately and cable separately to find problems. Megga test only the motor is half job.

strujic said:

He is not electrician maybe cooky

I always meggar motor from start panel and get readings of motor and cable. If readings not good open motor terminal box disconnect cable and meggar motor windings separately and cable separately to find problems. Megga test only the motor is half job.

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He is an electrician with over 40 years experience so cooky is not how I would describe him, rushed maybe but definitely not cooky.

If a motor is tripping out why would you Megga the cable and the motor together when in 95% of the time it is the motor.

IMHO if you Megga the motor and the cable and get a bad reading you have to Megga the motor anyway so why not go with what is probably bad in the first place to save you doing 2 tests, when in all probability you only have to do 1 test.

Mark

What exactly are you trying to accomplish?

Is this a new installation that has never been run before?

Is this a check for maintenance after xxx hours of running?

When checking the insulation resistance of any system, you must first understand what you are checking and why. 5 megohms of resistance might be OK, but for a new installation, I would expect more. Also, I would like to see a Polarization Index done on each system.

If this is a system that is running, then why are you checking it? If it was bad, the breaker would have tripped a long time ago. Checking a system after running will only yeild good results if you have a baseline data. Without that, you are wasting your time.

If you truly feel any voltage is OK to test with, then use a Fluke and put it on the ohms scale. Anythnig more and you need to disconnect the softstart and cables and only check the motor.

Also, if you don't like the advice here and want to argue when good sound advice is given, then I would suggest that you don't waste your or our time and not even ask the question, especially with all your experience.

BTW, the term Megger is a trademarked name by the Megger company, formally James G. Biddle Co. and AVO International. The proper terminology for what you are doing is Insulation Resistance Testing.

MarkNightingale said:

If a motor is tripping out why would you Megga the cable and the motor together when in 95% of the time it is the motor.

IMHO if you Megga the motor and the cable and get a bad reading you have to Megga the motor anyway so why not go with what is probably bad in the first place to save you doing 2 tests, when in all probability you only have to do 1 test.
Mark

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You don't have to necessarily wait for a something to start tripping to meg a motor. The point is to test the condition of the insulation. If you have equipment in a hot environment this is a good preventative action to be taken annually or semi-annually. Same as infrared testing it can catch problems before they cost a lot of money or downtime.

Yes you have to meg twice if you test the cable and motor together but it is also faster to disconnect the motor in the mcc or softstart cabinet and test both together. If you have a problem THEN find out which one. No point in taking the time to cut apart a junction on the motor if this a just a preventative.

brucechase said:

BTW, the term Megger is a trademarked name by the Megger company, formally James G. Biddle Co. and AVO International. The proper terminology for what you are doing is Insulation Resistance Testing.

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Which is very commonly referred to as megging a motor among electricians and other tradesmen

Insulation Testing

Why can't you experts answer the question rather than deviate off onto your own self riteous tangents? The question clearly is to check the insulation's resistence of a large number of motors within a short amount of time. Given this information this is obviously a programmed preventaive maintenance task. As we all know prevention is better than cure and by gathering and tracking data such as this and motor currents and vibration costly down time can be prevented by effecting a repair before it is required. If there was an actual fault on any one of these motors he would obviously follow a step by step trouble shooting sequence to determine where the fault is, motor/isolator/cable etc. I enjoy reading this forum but please give our less experienced friends the actual answer(s) to the questions that they are actually asking and the simple respect that they deserve. Thankyou

I normally check the motor and cables from the controller. If it is a motor starter, I don't disconnect it. I lock out the power source and ensure the control voltage is disabled so the contactor cannot close. If it is a solid state device (VFD or soft starter) I always disconnect it.

If a problem is found, then I move downstream to the local disconnect or motor junction box to determine if the problem is in a section of cabling or at the motor itself.

If no problem is found, then I know that the whole circuit (not just the motor) is good as "seen" by the controller.

This is usually a pretty quick way to test many motors in succession, but that depends on where the controllers are located and how much space you have to work with the wiring.

I have seen VFDs "die" due to megohmeter voltage done by inexperienced techs who left them connected, and I have seen them survive. I have not seen a soft starter "meggered", but I wouldn't personally take the chance.

Thanks Peanuts
This forum seems to have sunk a bit in the last few years.